1. Field of the Invention
The present invention relates to a power transmission apparatus for a four-wheel driven vehicle, and more specifically to a power transmission apparatus of viscous coupling type provided with a clutch mechanism.
2. Description of the Prior Art
Conventionally, there are various power transmission apparatus for a four-wheel driven vehicle, which can be classified according to presence or absence of a central differential gear device for absorbing a difference in revolution speed between front and rear wheel shafts, or the number or the arrangement condition of the differential limiting devices, etc.
FIG. 1 shows one of the prior-art power transmission apparatus, which is composed of a final reduction gear 205 driven by a drive gear 203 of a transmission 201; a front wheel differential gear device 207 rotated together with this final reduction gear 205; a direction converting gear assembly 209 driven by the front wheel differential gear device 207 to convert the rotational direction of the driving force at a right angle; and a viscous coupling device 213 provided for a transmission shaft to transmit the rotary force from the direction converting gear assembly 209 to the rear wheel differential gear device 211.
In the above-mentioned power transmission apparatus as described above, since no central differential gear device is incorporated to absorb a difference in rotational speed between the front and rear wheel shafts, there exists an advantage that the structure is simple. In addition, since power (torque) is transmitted to the rear wheel shaft (indirectly coupled wheel shaft) via the viscous coupling device 213, it is possible to automatically distribute torque to both the front and rear wheels in the same way as when two-wheel driving is switched to four-wheel driving or vice versa according to road and travel conditions.
In the prior-art power transmission apparatus, however, since a big difference in revolution speed between locked side (e.g. front) wheel shaft and non-locked side (e.g. rear) wheel shaft will be produced when an emergency brake is applied to the four wheel driven vehicle, a large braking force is inevitably transmitted from the locked side (e.g. front) wheel shaft to the non-locked side (e.g. rear) wheel shaft on the basis of the characteristics of the viscous coupling. This problem is disadvantageous in an antiskid braking system (ABS) for obtaining excellent brake characteristics by applying an appropriate braking force to each wheel shaft; that is, there exists a problem in that it is impossible to operate the antiskid braking system reliably in cooperation with the power transmission apparatus of viscous coupling type.
On the other hand, when the vehicle is running at high speed for many hours under the condition that tire inflation pressure is different between the front and rear wheels, since a big difference in revolution speed will be produced between the front and rear wheels, there exists another problem in that a large load is applied to the viscous coupling and therefore the temperature of silicon oil within the fluid working chamber rises abnormally and thus the viscous coupling may be damaged.